IRCClient fails to parse non-UTF8 messages under Python 3

Words' IRC client fails internally under Python 3 (#6320) because it
attempts to decode all incoming data as utf-8. While the protocol
appears to be ASCII, RFC1459 does not specify an encoding for the
contents of PRIVMSGs or channel topics. I ran a naive a survey of the
10 most popular networks, as found here:

A user cannot register an errback to handle this because the exception
occurs as a result of a read event. They also can't write code that
doesn't encounter this exception without monkeypatching Twisted.

I believe this is a serious regression and the IRC port deserves more
attention.